U.S. patent application number 14/749770 was filed with the patent office on 2016-12-29 for perpendicular magnetic anisotropy free layers with iron insertion and oxide interfaces for spin transfer torque magnetic random access memory.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Guohan Hu.
Application Number | 20160380188 14/749770 |
Document ID | / |
Family ID | 57602857 |
Filed Date | 2016-12-29 |
United States Patent
Application |
20160380188 |
Kind Code |
A1 |
Hu; Guohan |
December 29, 2016 |
PERPENDICULAR MAGNETIC ANISOTROPY FREE LAYERS WITH IRON INSERTION
AND OXIDE INTERFACES FOR SPIN TRANSFER TORQUE MAGNETIC RANDOM
ACCESS MEMORY
Abstract
A method of making a spin-torque transfer magnetic random access
memory device (STT MRAM) device includes forming a tunnel barrier
layer on a reference layer; forming a free layer on the tunnel
barrier layer, the free layer comprising a cobalt iron boron
(CoFeB) alloy layer and an iron (Fe) layer; and performing a
sputtering process to form a metal oxide layer on the Fe layer.
Inventors: |
Hu; Guohan; (Yorktown
Heights, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
57602857 |
Appl. No.: |
14/749770 |
Filed: |
June 25, 2015 |
Current U.S.
Class: |
438/3 |
Current CPC
Class: |
H01L 43/10 20130101;
H01L 43/02 20130101; H01L 43/12 20130101; H01L 43/08 20130101 |
International
Class: |
H01L 43/12 20060101
H01L043/12; H01L 43/10 20060101 H01L043/10; H01L 43/08 20060101
H01L043/08 |
Claims
1-8. (canceled)
9. A method of making a STT MRAM device, the method comprising:
forming a tunnel barrier layer on a reference layer; forming a free
layer on the tunnel barrier layer, the free layer comprising a
magnetic layer, a CoFeB alloy layer, a spacer layer between the
magnetic layer and the CoFeB alloy layer, and an Fe layer on the
CoFeB alloy layer; and performing a sputtering process to form a
metal oxide layer on the free layer.
10. The method of claim 9, wherein the CoFeB alloy layer comprises
boron (B) in an amount in a range from about 20 to about 30 at.
%.
11. The method of claim 9, wherein the CoFeB alloy layer comprises
Fe in an amount in a range from about 20 to about 60 at. %.
12. The method of claim 9, wherein the CoFeB alloy layer comprises
cobalt (Co) in an amount in a range from about 20 to about 40 at.
%.
13. The method of claim 9, wherein the sputtering process is RF
sputtering.
14. The method of claim 13, wherein the RF sputtering comprises
sputtering a metal oxide onto the CoFeB alloy layer.
15. The method of claim 14, wherein the metal oxide is MgO,
tantalum oxide (TaOx), titanium oxide (TiOx), aluminum oxide
(AlOx), magnesium titanium oxide (MgTiOx), or any combination
thereof.
16. A method of making a STT MRAM device, the method comprising:
forming a tunnel barrier layer on a reference layer; forming a free
layer on the tunnel barrier layer, the free layer comprising a
magnetic layer, a CoFeB alloy layer, a spacer layer between the
magnetic layer and the CoFeB alloy layers, and an Fe layer disposed
on the CoFeB alloy layer; and performing a sputtering process to
form a metal oxide layer on the free layer; wherein sputtering
process comprises RF sputtering a metal oxide onto the Fe layer
under a pressure in a range from about 0.1 to about 10 milli-Torr
(mTorr).
17. The method of claim 16, wherein the Fe layer comprises at least
99 at. % Fe.
18. The method of claim 16, wherein the free layer has a thickness
in a range from about 0.6 to about 6 nm.
19. The method of claim 16, wherein the metal oxide layer has a
thickness in a range from about 0.2 to about 2 nm.
20. The method of claim 16, wherein the sputtering process further
comprises depositing the metal oxide at a deposition rate in a
range from about 0.0005 nm/second to 0.005 nm/second.
Description
BACKGROUND
[0001] The present invention generally relates to spin-transfer
torque magnetic random access memory (STT MRAM) devices, and more
specifically to perpendicular magnetic anisotropy (PMA) materials
in STT MRAM devices.
[0002] A STT MRAM device is a type of solid state, non-volatile
memory device that uses tunneling magnetoresistance (TMR or MR) to
store information. MRAM includes an electrically connected array of
magnetoresistive memory elements, referred to as magnetic tunnel
junctions (MTJs). Each MTJ includes a free layer and
fixed/reference layer that each include a magnetic material. The
free and fixed/reference layers are separated by a non-magnetic
insulating tunnel barrier. The free layer and the reference layer
are magnetically decoupled by the tunnel barrier. The free layer
has a variable magnetization direction, and the reference layer has
an invariable magnetization direction.
[0003] The MTJ stores information by switching the magnetization
state of the free layer. When the free layer's magnetization
direction is parallel to the reference layer's magnetization
direction, the MTJ is in a low resistance state. Conversely, when
the free layer's magnetization direction is antiparallel to the
reference layer's magnetization direction, the MTJ is in a high
resistance state. The difference in resistance of the MTJ indicates
a logical `1` or `0`, thereby storing a bit of information. The TMR
of an MTJ determines the difference in resistance between the high
and low resistance states. A relatively high difference between the
high and low resistance states facilitates read operations in the
MRAM.
[0004] The magnetization direction of the free layer may be changed
by a spin-transfer torque (STT) switched write method, in which a
write current is applied in a direction perpendicular to the film
plane of the magnetic films forming the MTJ. The write current
transfers spin angular momentum to the free layer which creates a
torque to change (or reverse) the free layer's magnetization
direction. During STT magnetization reversal, the write current for
magnetization reversal is determined by the current density. As the
surface area of the the MTJ becomes smaller, the write current for
reversing the free layer's magnetization becomes smaller.
Therefore, if writing is performed with fixed current density, the
necessary write current becomes smaller as the MTJ size becomes
smaller.
[0005] Compared to MTJs with in-plane magnetic anisotropy, layers
with perpendicular magnetic anisotropy (PMA) can lower the
necessary write current density. Thus, PMA materials lower the
total write current used.
SUMMARY
[0006] In one embodiment of the present invention, a method of
making a spin-torque transfer magnetic random access memory device
(STT MRAM) device includes forming a tunnel barrier layer on a
reference layer; forming a free layer on the tunnel barrier layer,
the free layer comprising a cobalt iron boron (CoFeB) alloy layer
and an iron (Fe) layer; and performing a sputtering process to form
a metal oxide layer on the Fe layer.
[0007] In another embodiment, a method of making a STT MRAM device
includes forming a tunnel barrier layer on a reference layer;
forming a free layer on the tunnel barrier layer, the free layer
comprising a magnetic layer, a CoFeB alloy layer, a spacer layer
between the magnetic layer and the CoFeB alloy layer, and an Fe
layer on the CoFeB alloy layer; and performing a sputtering process
to form a metal oxide layer on the free layer.
[0008] Yet, in another embodiment, a method of making a STT MRAM
includes forming a tunnel barrier layer on a reference layer;
forming a free layer on the tunnel barrier layer, the free layer
comprising a magnetic layer, a CoFeB alloy layer, a spacer layer
between the magnetic layer and second CoFeB alloy layers, and an Fe
layer disposed on the second CoFeB alloy layer; and performing a
sputtering process to form a metal oxide layer on the free layer;
wherein sputtering process comprises RF sputtering a metal oxide
onto the Fe layer under a pressure in a range from about 0.1 to
about 10 milli-Torr (mTorr).
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The subject matter which is regarded as the invention is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The forgoing and other
features, and advantages of the invention are apparent from the
following detailed description taken in conjunction with the
accompanying drawings in which:
[0010] FIGS. 1-3 illustrate various embodiments of STT MRAM devices
according to the present invention, in which:
[0011] FIG. 1 illustrates a cross-sectional side view of a STT MRAM
device utilizing CoFeB layers with PMA;
[0012] FIG. 2 illustrates a cross-sectional side view of a STT MRAM
device with a free layer including an CoFeB layer and an Fe
layer;
[0013] FIG. 3 illustrates a cross-sectional side view of a STT MRAM
device with a free layer including a magnetic layer, a spacer
layer, a CoFeB layer, and an Fe layer;
[0014] FIG. 4 is a flow diagram illustrating a method of forming a
STT MRAM device;
[0015] FIG. 5A is a graph comparing switching efficiency
(E.sub.b/I.sub.c0) in STT MRAM devices with free layers including
CoFeB alone and a combination of CoFeB and Fe layers; and
[0016] FIG. 5B is a graph comparing energy per bit (E.sub.b) in STT
MRAM devices with free layers including CoFeB alone and a
combination of CoFeB and Fe layers.
DETAILED DESCRIPTION
[0017] As stated above, the present invention relates to STT MRAM
devices, and more specifically to PMA materials in STT MRAM
devices. It is noted that like reference numerals refer to like
elements across different embodiments.
[0018] The following definitions and abbreviations are to be used
for the interpretation of the claims and the specification. As used
herein, the terms "comprises," "comprising," "includes,"
"including," "has," "having," "contains" or "containing," or any
other variation thereof, are intended to cover a non-exclusive
inclusion. For example, a composition, a mixture, process, method,
article, or apparatus that comprises a list of elements is not
necessarily limited to only those elements but can include other
elements not expressly listed or inherent to such composition,
mixture, process, method, article, or apparatus.
[0019] As used herein, the articles "a" and "an" preceding an
element or component are intended to be nonrestrictive regarding
the number of instances (i.e. occurrences) of the element or
component. Therefore, "a" or "an" should be read to include one or
at least one, and the singular word form of the element or
component also includes the plural unless the number is obviously
meant to be singular.
[0020] As used herein, the terms "invention" or "present invention"
are non-limiting terms and not intended to refer to any single
aspect of the particular invention but encompass all possible
aspects as described in the specification and the claims.
[0021] As used herein, the term "about" modifying the quantity of
an ingredient, component, or reactant of the invention employed
refers to variation in the numerical quantity that can occur, for
example, through typical measuring and liquid handling procedures
used for making concentrates or solutions. Furthermore, variation
can occur from inadvertent error in measuring procedures,
differences in the manufacture, source, or purity of the
ingredients employed to make the compositions or carry out the
methods, and the like. In one aspect, the term "about" means within
10% of the reported numerical value. In another aspect, the term
"about" means within 5% of the reported numerical value. Yet, in
another aspect, the term "about" means within 10, 9, 8, 7, 6, 5, 4,
3, 2, or 1% of the reported numerical value.
[0022] As used herein, the terms "atomic percent," "atomic %" and
"at. %" mean the percentage of atoms of a pure substance divided by
the total number of atoms of a compound or composition, multiplied
by 100.
[0023] As used herein, the term "magnetic anisotropy" means the
magnetization prefers to orient in a particular direction.
[0024] As used herein, the terms "perpendicular magnetic
anisotropy" and "PMA" mean the magnetization prefers to orient
perpendicular to the xy-plane. PMA can be determined by measuring
magnetic hysteresis loops in both in-plane and out-of-plane
directions.
[0025] Magnetic materials with PMA are useful for free layer
applications in STT MRAM devices. However, STT devices with
magnetic materials having both sufficiently strong PMA at a low
switching current are a challenge.
[0026] Accordingly, the present invention solves the above problem
by providing a method of making a STT MRAM device with desirable
magnetic materials. The methods described provide STT MRAM devices
with magnetic materials having sufficiently strong PMA at a low
switching current (e.g., at least 50% higher switching efficiency
compared to STT MRAM devices with free layers of CoFeB alone).
[0027] In particular, the inventive methods utilize sputtering
processes to form an oxide cap over the free layer. The sputtering
processes provide advantages of increased control over the oxide
layer thickness and, therefore, improved control of the junction
resistance-area (RA) product, as well as the distribution of the RA
in patterned devices. Compared to initially depositing a metal
layer and then oxidizing the metal layer by an oxidation process,
as described herein, the oxide cap layer is formed by RF sputtering
from an oxide target.
[0028] Turning now to the Figures, FIGS. 1-3 illustrate various
embodiments of STT MRAM devices according according to the present
invention. FIG. 1 illustrates a cross-sectional side view of a STT
MRAM device utilizing magnetic layers with PMA. The STT MRAM device
includes a magnetic tunnel junction (MTJ) 122 over a seed layer
110. The MTJ 122 includes a reference layer 120, a tunnel barrier
layer 130 on the reference layer 120, and a free layer 140 on the
tunnel barrier layer 130.
[0029] The reference layer 120 and the free layer 140 are magnetic
materials. The free layer 140 is described in further detail in
FIGS. 2 and 3 below. The reference layer 120 may include any metal
or metal alloys. Non-limiting examples of suitable materials for
the reference layer 120 include cobalt (Co), iron (Fe), boron (B),
nickel (Ni), iridium (Ir), platinum (Pt), palladium (Pd), or any
combination thereof.
[0030] The thickness of the reference layer 120 is not intended to
be limited. In one aspect, the thickness of the reference layer 120
is in a range from about 10 nanometers (nm) to about 20 nm. In
another aspect, the thickness of the reference layer 120 is in a
range from about 2 nm to about 10 nm. Yet, in another aspect, the
thickness of the reference layer 120 is about or in any range from
about 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 nm.
[0031] The free layer 140 is shown with double arrows to illustrate
that spin torque current (or spin-polarized current) via voltage
source 170 can flip the magnetic orientation of the free layer 140
to up or down as desired. The reference layer 120 is shown with an
up arrow to illustrate a magnetic orientation in the up direction.
To write the STT-RAM device, the voltage source 170 applies voltage
such that a spin torque current may flip the magnetic orientation
of the free magnetic layer 140 as desired. When the magnetic
orientations of the free layer 140 and the reference layer 120 are
parallel (i.e., pointing in the same direction), the resistance of
the MTJ 122 is low (e.g., representing logic 0). When the magnetic
orientations of the free layer 140 and the reference layer 120 are
antiparallel (i.e., pointing in opposite directions), the
resistance of the MTJ 122 is high (e.g., representing a logic
1).
[0032] One non-limiting example of a suitable material for the
tunnel barrier layer 130 includes magnesium oxide (MgO). Any
insulating material may be used in the tunnel barrier layer 130.
The thickness of the tunnel barrier layer 130 is not intended to be
limited. In one aspect, the thickness of the tunnel barrier layer
130 is in a range from about 0.5 nm to about 2 nm.
[0033] The seed layer 110 may include one or more different
materials, depending on the composition of the reference layer 130,
to grow the reference layer 120. Non-limiting examples of suitable
materials for the seed layer 110 include NiCr, Ta, TaN, Pt, Pd, Ru,
Ir, or any combination thereof. The thickness of the seed layer 110
is not intended to be limited. In one aspect, the thickness of the
seed layer 110 is in a range from about 5 nm to about 10 nm. In
another aspect, the thickness of the seed layer 110 is in a range
from about 1 nm to about 5 nm. Yet, in another aspect, the
thickness of the seed layer 110 is about or in any range from about
1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 nm.
[0034] A metal oxide layer 150 is formed on the free layer 140. The
metal oxide layer 150 is formed by a sputtering process. In one
aspect, the sputtering process is a radio frequency (RF) sputtering
process. During RF sputtering, a metal oxide is sputtered onto the
free layer 140. Non-limiting examples of suitable metal oxides for
forming the metal oxide layer 140 include MgO, tantalum oxide
(TaOx), titanium oxide (TiOx), aluminum oxide (AlOx), magnesium
titanium oxide (MgTiOx), or any combination thereof. The thickness
of the metal oxide layer 150 is not intended to be limited. In one
aspect, the thickness of the metal oxide layer 150 is in a range
from about 0.2 to about 2 nm. Yet, in another aspect, the thickness
of the metal oxide layer 150 is about or in any range from about
0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 nm.
[0035] RF sputtering is performed under a pressure in a range from
about 0.1 mTorr to about 10 mTorr. The deposition rate of the oxide
cap material is controlled in a range from about 0.0005 nm/second
to 0.005 nm/second. The low sputter rate provides precise control
of the cap oxide layer thickness. The oxide material sputtered from
the oxide target can be controlled provide the right stoichiometry
without excessive oxygen or oxygen deficiency. Such control
minimizes the interaction between the free layer material and oxide
cap by taking advantage of the interface anisotropy. In contrast,
when the oxide cap is formed by metal deposition followed by
subsequent oxidation, it is difficult to control the oxidation
process to provide stoichiometric oxide and to provide oxygen stops
exactly at the free layer interface. As a result, the free layer
can be easily oxidized and causes high RA, which is
unfavorable.
[0036] Cap layer 160 is formed over the metal oxide layer 150.
Non-limiting examples of suitable materials for the metal oxide
layer 150 include ruthenium (Ru), Pd, Pd, Pt, Ta, titanium nitride
(TiN), or any combination thereof. The thickness of the cap layer
160 is not intended to be limited. In one aspect, the thickness of
the cap layer 160 is in a range from about 1 nm to about 10 nm.
[0037] The seed layer 110, reference layer 120, tunnel barrier
layer 130, free layer, and cap layer 160 may be formed by any
suitable deposition processes. Non-limiting examples of suitable
deposition processes include physical vapor deposition (PVD),
chemical vapor deposition (CVD), high density plasma CVD (HDP CVD),
epitaxial growth, or other suitable deposition processes.
[0038] FIG. 2 illustrates a cross-sectional side view of a STT MRAM
device with a free layer 140 including a CoFeB alloy layer 210 and
an Fe layer 220. The STT MRAM device includes a MTJ 122 over a seed
layer 110. The MTJ 122 includes a reference layer 120, a tunnel
barrier layer 130 on the reference layer 120, and a free layer 140
on the tunnel barrier layer 130. The free layer 140 includes a
CoFeB alloy layer 210 and an Fe layer 220 on the CoFeB alloy layer
210. The CoFeB alloy layer 210 and the Fe layer 220 are discrete
layers that are strongly ferromagnetically coupled and switch as a
single entity under spin torque currents. The Fe layer 220
substantially improves the PMA of the free layer 140.
[0039] The CoFeB alloy layer 210 includes boron (B) in amount in a
range from about 5 to about 50 at. %. In another aspect, the CoFeB
layer 210 include boron in an amount in a range from about 20 to
about 30 at. %. Yet, in another aspect, the CoFeB alloy layer 210
includes boron in an amount about or in any range from about 5, 10,
15, 20, 25, 30, 35, 40, 45, and 50 at. %.
[0040] The CoFeB alloy layer 210 includes iron (Fe) in amount in a
range from about 20 to about 80 at. %. In another aspect, the CoFeB
layer 210 include iron in an amount in a range from about 20 to
about 60 at. %. Yet, in another aspect, the CoFeB alloy layer 210
includes iron in an amount about or in any range from about 20, 25,
30, 35, 40, 45, 50, 55, 60, 65, 70, 75, and 80 at. %.
[0041] The CoFeB alloy layer 210 includes cobalt (Co) in amount in
a range from about 10 to about 50 at. %. In another aspect, the
CoFeB layer 210 include cobalt in an amount in a range from about
20 to about 30 at. %. Yet, in another aspect, the CoFeB alloy layer
210 includes cobalt in an amount about or in any range from about
10, 15, 20, 25, 30, 35, 40, 45, and 50 at. %.
[0042] The thickness of the CoFeB alloy layer 210 is not intended
to be limited. In one aspect, the thickness of the CoFeB alloy
layer 210 is in a range from about 0.2 nm to about 3 nm. In another
aspect, the thickness of the CoFeB alloy layer 210 is in a range
from about 0.5 to about 2 nm. Yet, in another aspect, the thickness
of the CoFeB alloy layer 210 is about or in any range from about
0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6,
2.8, and 3 nm.
[0043] In one aspect, the Fe layer 220 includes at least 98 at. %
Fe. In another aspect, the Fe layer 220 includes at least 99 at. %
Fe. Yet, in another aspect, the Fe layer 220 is substantially pure
Fe. The Fe layer 220 may include other additional metals or
non-metals. The thickness of the Fe layer 220 is not intended to be
limited. In one aspect, the thickness of the Fe layer 220 is in a
range from about 0.2 to about 2 nm. In another aspect, the
thickness of the Fe layer 220 is in a range from about 0.2 to about
1.5 nm. Yet, in another aspect, the thickness of the Fe layer 220
is about or in any range from about 0.2, 0.4, 0.6, 0.8, 1.0, 1.2,
1.4, 1.6, 1.8, and 2.0 nm.
[0044] The CoFeB alloy layer 210 and the Fe layer 220 may be formed
by any suitable deposition processes. Non-limiting examples of
suitable deposition processes include PVD, CVD, and HDP CVD.
[0045] The metal oxide layer 150 is formed by sputtering (e.g., RF
sputtering) of a metal oxide onto the Fe layer 220 as described
above for FIG. 1. The cap layer 160 is formed on the metal oxide
layer 150.
[0046] FIG. 3 illustrates a cross-sectional side view of a STT MRAM
device with a free layer 140 including a magnetic layer 310, a
spacer layer 320, a CoFeB layer 210, and an Fe layer 220. The STT
MRAM device includes a MTJ 122 over a seed layer 110. The MTJ 122
includes a reference layer 120, a tunnel barrier layer 130 on the
reference layer 120, and a free layer 140 on the tunnel barrier
layer 130. The free layer 140 includes a magnetic layer 310, a
CoFeB alloy layer 210, a spacer layer 320 between the magnetic
layer 310 and the CoFeB alloy layer 210, and a Fe layer 220 on the
CoFeB layer 210.
[0047] The magnetic layer 310 and the CoFeB alloy layer 210, which
is also magnetic, are ferromagnetically coupled through the spacer
layer 320. When a voltage source (see FIG. 1) generates the spin
torque current (spin polarized current), the magnetic orientations
(maintained in the same direction with respect to one another) of
the magnetic layer 310 and the CoFeB alloy layer 210 are both
flipped in the same direction according to the angular momentum of
the spin torque current. Accordingly, when the free layer 140 is
parallel to the reference layer 120, the resistance is low and the
logic state is 0. When the free layer 140 is antiparallel to the
reference layer 120, the resistance is high and the logic state is
1.
[0048] Non-limiting examples of suitable materials for the magnetic
layer 310 include cobalt (Co), iron (Fe), boron (B), nickel (Ni),
or any combination thereof. The thickness of the magnetic layer 310
is not intended to be limited. In one aspect, the thickness of the
magnetic layer 310 is in a range from about 0.2 nm to about 2 nm.
In another aspect, the thickness of the magnetic layer 310 is in a
range from about 0.5 to about 1 nm. Yet, in another aspect, the
thickness of the magnetic layer 310 is about or in any range from
about 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0.
[0049] Non-limiting examples of suitable materials for the spacer
layer 320 include tantalum (Ta), titanium (Ti), titanium nitride
(TiN), aluminum (Al), magnesium (Mg), titanium magnesium (TiMg),
tantalum magnesium (TaMg), or any combination thereof. The
thickness of the spacer layer 320 is not intended to be limited. In
one aspect, the thickness of the spacer layer 320 is in a range
from about 0.1 to about 1 nm. In another aspect, the thickness of
the spacer layer 320 is in a range from about 0.2 to about 0.5 nm.
Yet, in another aspect, the thickness of the spacer layer 320 is
about or in any range from about 0.1, 0.2, 0.3, 0.4, 0.6, 0.6, 0.7,
0.8, 0.9, and 1.0 nm.
[0050] The metal oxide layer 150 is formed by sputtering (e.g., RF
sputtering) of a metal oxide onto the Fe layer 220 as described
above for FIG. 1. The cap layer 160 is formed on the metal oxide
layer 150.
[0051] The magnetic layer 310 and the spacer layer 320 may be
formed by any suitable deposition processes. Non-limiting examples
of suitable deposition processes include PVD, CVD, and HDP CVD.
[0052] FIG. 4 is a flow diagram illustrating a method of forming a
STT MRAM device (see also, FIGS. 1-3). In box 410, the method
includes forming a tunnel barrier layer on a reference layer. In
box 420, a free layer is formed on the tunnel barrier layer. The
free layer includes a CoFeB alloy layer and an Fe layer. In box
430, a sputtering process is used to form a metal oxide layer on
the Fe layer. The methods described in boxes 410, 420, and 430 are
described in further detail above for FIGS. 1-3.
Example
[0053] FIG. 5A is a graph comparing switching efficiency
(E.sub.b/I.sub.c0) in STT MRAM devices with free layers including
CoFeB alone (510) and the combination of CoFeB and Fe (520). As
shown, switching efficiency was improved by 50% in the CoFeB/Fe
free layer device (520 compared to 510) at a given device size (see
FIG. 1B). FIG. 5B is a graph comparing energy per bit (E.sub.b) in
STT MRAM devices with free layers including CoFeB alone (530) and
the combination of CoFeB and Fe (540). As shown, both devices
demonstrated similar energy per bit.
[0054] The above described STT MRAM devices and methods provide
various advantages. The devices with free layers with a CoFeB and
Fe layer combination and a metal oxide over the free layer
demonstrate sufficiently strong PMA at a low switching current
(e.g., at least 50% higher switching efficiency compared to STT
MRAM devices with free layers of CoFeB alone). The method of making
the devices, including forming a metal oxide layer over the free
layer by sputtering provides advantages of increased control over
the oxide layer thickness and, therefore, improved control of the
junction resistance-area (RA) product, as well as the distribution
of the RA in patterned devices. Compared to initially depositing a
metal layer and then oxidizing the metal layer by an oxidation
process, as described herein, the oxide cap layer is formed by RF
sputtering from an oxide target.
[0055] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, element components, and/or groups thereof.
[0056] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. Many modifications and variations
will be apparent to those of ordinary skill in the art without
departing from the scope and spirit of the invention. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
[0057] The flow diagrams depicted herein are just one example.
There may be many variations to this diagram or the steps (or
operations) described therein without departing from the spirit of
the invention. For instance, the steps may be performed in a
differing order or steps may be added, deleted or modified. All of
these variations are considered a part of the claimed
invention.
[0058] The descriptions of the various embodiments of the present
invention have been presented for purposes of illustration, but are
not intended to be exhaustive or limited to the embodiments
disclosed. Many modifications and variations will be apparent to
those of ordinary skill in the art without departing from the scope
and spirit of the described embodiments. The terminology used
herein was chosen to best explain the principles of the
embodiments, the practical application or technical improvement
over technologies found in the marketplace, or to enable others of
ordinary skill in the art to understand the embodiments disclosed
herein.
* * * * *